BY H. I. JENSEN. 577 



At present we have only the agency of erosion tending steadily 

 by means of sand blast (wind) action, rainfall, etc., to reduce the 

 mountains to the level of the western plains. 



(f) Remarks,— The peneplain which developed in early Tertiary 

 or late Cretaceous times, before the commencement of volcanic 

 activity, may have been shaped by arid agencies like those 

 prevailing at present. However, evidence in favour of this 

 supposition is weak, and only of a negative character, consisting 

 in the fact that we find no marine or estuarine fossils of Cretaceo- 

 Eocene age anywhere within a very large area, such as we would 

 expect on submerged parts of a true peneplain. 



The slight partial dissection of this peneplain which preceded 

 the phonolitic series of eruptions may, if the peneplain were of 

 arid origin, have been due either to an uplift caused by intrusion 

 of sills and laccolites, or by the development of an exterior 

 drainage leading to a renewal of erosion. 



' The most puzzling problem met with in the field was that of 

 the relative age of the light-coloured and dark-coloured trachytes. 

 The structure at Timor Rock, which I have already discussed, 

 and that observed at Paddy's Rock in the Naman Ledges opposite 

 Black Mountain, where we have a mass of grey arfvedsouite 

 trachyte surrounded by a narrow rim of tilted sandstone around 

 which there are undisturbed flows of dark trachyte, may be 

 explained in two ways. The neck of arfvedsonite trachyte may 

 be imagined to be a plug which has filled the vent through which 

 the aegirine trachyte rose and flowed over the country; or it may 

 be imagined to be an earlier mamelon or neck surrounded by 

 later flows of more basic rock. I have, however, nowhere seen 

 arfvedsonite trachyte or its tuffs overlying the more basic rock 

 in a flow or sheet. It reaches higher elevations, but either over- 

 lies sandstone or extends down to unknown depths. The more 

 basic trachytes, however, have been noticed in numerous places 

 capping the arfvedsonite trachytes in flows and sheets, as at 

 Goat Mountain near Tannabar (Fig. 7), Mt. Caraghnan and 

 Uargon Creek, etc. / have therefore come to the conclusion that 

 the dark trachytes are the newest, and that many of the ridges of 



